1. Field of the Invention
The present invention relates to an optical glass having optical constants in the form of a refractive index (nd) of greater than or equal to 1.65 and an Abbé number (ν (nu)d) of greater than or equal to 50; a preform for precision press molding comprised of said glass; a method for manufacturing said preform; and a method for manufacturing optical elements comprised of said glass.
2. Discussion of the Background
The arrival of digital cameras and portable cellular phones equipped with cameras has resulted in the rapid progression of high integration and high functionality in devices employing optical systems. At the same time, the demand for high precision, light weight, compact optical systems has been intensifying.
In recent years, to satisfy these demands, optical designs employing aspherical lenses have increasingly moved into the mainstream. Thus, to stably and economically provide large quantities of aspherical lenses employing highly functional glass, precision press molding techniques (also known as mold optics molding techniques) in which optically functional surfaces are molded directly by press molding without grinding or polishing steps have attracted attention. The demand for optical glass having low temperature softening properties suited to precision press molding is increasing each year. Such glasses include high refractive index and low dispersion glasses with a refractive index (nd) of greater than or equal to 1.65 and an Abbé number (nud) of greater than or equal to 50. An example of a high refractive index and low dispersion glass having low temperature softening properties is described in Japanese Patent No. 2,616,958, which is expressly incorporated herein by reference in their entirety.
In the course of manufacturing optical elements such as lenses by precision press molding, when the glass is precision press molded, damage sometimes occurs during cooling in the pressing mold. Such damage consists of crizzles and cracking. This is particularly prominent in the high refractive index and low dispersion glasses referred to above, and compromises productivity.
Given these circumstances, the present invention has for its object to provide means of manufacturing with high productivity optical elements comprised of high refractive index and low dispersion glass by precision press molding.
The present inventors extensively investigated the causes of the above-described crizzles and cracking and discovered the following.
In precision press molding, once the glass has been pressed into a desired shape in a pressing mold, the glass is maintained within the pressing mold, which is kept closed, so that the glass surface retains the shape transferred by the mold surface while being cooled to a temperature range at which the glass tends not to deform. At that time, the portion of the glass in proximity to the surface cools rapidly, but the center portion of the glass cools slowly. Thus, even after the portion near the surface has reached a temperature below the glass transition temperature, the temperature at the center is still higher than the glass transition temperature. When the present inventors examined expansion properties relating to conventional glasses prone to crizzles and cracking, they found that the ratio of the coefficients of linear thermal expansion above and below the glass transition temperature was considerably higher than that of glasses not prone to crizzles and cracking. That is, in such glasses, when the temperature near the surface was lower than the glass transition temperature and the temperature at the center was higher than the glass transition temperature, there was much greater contraction of the center portion than of the portion in the vicinity of the surface. This phenomenon occurred when the glass was enclosed within the pressing mold, producing great stress in glass that had lost the ability to undergo plastic deformation. When the glass is firm in structure, no damage occurs even when stress is generated. However, the relatively weak structure of high refractive index and low dispersion glass falling within the above-stated optical constant ranges is thought to result in crizzles and cracking.
Accordingly, when the present inventors conducted further research based on these discoveries, they found that reducing the difference between the coefficients of expansion above and below the glass transition temperature in conventional high refractive index and low dispersion glass reduced the difference in the degree of contraction near the surface and in the center of the glass during the cooling step, thereby inhibiting crizzles and cracking. The present invention was devised on this basis.
The present invention provides an optical glass that tends not to be damaged during precision press molding, a preform for precision press molding that is comprised of said glass, and an optical element that is comprised of said glass.
Further, the present invention permits the manufacturing of optical elements at high productivity without damaging the glass.